[MODULE] Assert that module ref. count is > 0 in module uninitialization.
[DOUBLEBUFFER] Optimize double-buffering.

 * Isolate double-buffering into standalone source.
 * Make call of the control-specific paint callback inlined.
   (For modern CPUs, dealing with function pointers often
   leads to pipe line stall. Hence lets avoid it when possible).
 * Adapted all call sites.
[Misc] Use flag LOAD_LIBRARY_SEARCH_SYSTEM32...

... when loading system DLL (and when the flag is supported).